The invention relates to a laminated layer structure comprising a substrate, and a stack of a plurality of layers of a material comprising at least two compounds A and B in the layers wherein compound A has a lattice parameter being sufficient to allow a homo- or heteroepitaxial growth of compound A on the substrate, and wherein at least a part of the layers of the stack has a grading composition AxgB(1-xg), with x being a composition parameter within the range of 0 and 1 and with the composition parameter (1-xg) increasing gradually, in particular linearly, over the thickness of the corresponding layer. The invention also relates to a method for forming such a laminated layer structure.
This type of laminated structure is generally known from U.S. Pat. No. 6,525,338 where it is used as a starting material for obtaining a strained silicon substrate. Strained silicon substrates find their use in high-speed microelectronic devices. The strained silicon technology takes advantage of the tendency for atoms inside compounds to align with one another. Silicon is deposited on top of a substrate, the atoms of which are spaced farther apart than compared to the ones of silicon. To adapt to the structure of the substrate the atoms in the silicon layer will stretch to line up with the atoms beneath, thereby straining the silicon. In the strained silicon layer, the electrons experience less resistance to movement and this leads to an increase in speed in the microelectronic chips fabricated with strained silicon substrates.
Conventionally strained silicon is achieved by epitaxially growing silicon (Si) on a SixGe1-x layer that has a larger lattice constant, which itself has been epitaxially deposited on a Si substrate. Due to the difference in lattice parameters in the different layers, crystalline defaults, like dislocations and high surface roughness, are created, thereby limiting the crystalline quality of the strained silicon layer. These defaults limit the applicability of the strained silicon substrates. To overcome this problem, U.S. Pat. No. 6,525,338 proposes to provide a silicon germanium buffer layer on a starting silicon substrate, wherein the buffer layer is constructed out of a plurality of laminated layers comprising alternating layers of a silicon germanium gradient composition, in which the germanium composition increases gradually from the composition of the base material, and a silicon germanium constant composition layer provided on top of the gradient composition layer, in which the germanium composition ratio is equal to the one of the upper surface of the gradient composition layer. By providing alternating layers, an improved strained silicon layer was achieved with a surface roughness reduced down to 16 Å RMS and a dislocation density of 3.5×105 cm−2.
With the ever-increasing necessity of improved semiconductor substrates for the microelectronic industry, however, the achieved quality improvements of the strained silicon substrate according to U.S. Pat. No. 6,525,338 are not sufficient. There is therefore a need to improve such laminated layer structures to further reduce the surface roughness and the dislocation density.